Over the last few years, it has become possible to confine and guide light in micrometer-scale hollow-core waveguides based on photonic crystal structures like photonic crystal fiber (HC-PCF) (Russell, P., Laser Focus World 38: 77-82, 2002), omniguides (Fink, Y. et al. Science 282: 1679-1682, 1998), and Bragg waveguides (Hadley, et al., Opt. Lett. 29: 809-811, 2004). Benefits of this approach in the case of gas and vapor phase media include the miniaturization and simplification of existing measurement apparatuses, and—perhaps even more significant—the prospect of adding integrated optical components developed for all-solid photonics to implement new functionalities. Nonlinear optical devices are particularly attractive because the use of a waveguide eliminates the tradeoff between small beam areas and finite focal depth. This allows large intensities to be maintained over long distances. Consequently, there are numerous potential applications of hollow-core waveguide based atomic and molecular spectroscopy, including gas phase sensing, precision spectroscopy (Hansch, T W. et al., Phil. Trans. Royal Soc. London A 363: 2155-2163, 2005), atomic clocks (Knappe, S. et al. Opt. Lett. 30: 2351-2353, 2005), nonlinear frequency generation (Benabid, et al., Phys. Rev. Lett. 95, 213903, 2005), low-level all-optical switching (Dawes, et al., Science 308, 672-674, 2005), slow light (Lukin, M D. Rev Mod Phys 75:457-72, 2003; Hau, et al., Nature 397:594-598, 1999), and quantum communications (Eisaman M D. et al., Nature, 438: 837-841, 2005; Kolchin, et al., Phys. Rev. Lett. 97:113602, 2006). The latter areas are examples of the use of electromagnetically induced transparency (EIT) (Harris, S E., Phys. Today 50: 36-42, 1997)—extremely strong linear and nonlinear light-matter interactions that result from quantum interference effects. Alkali metal vapors are ideally suited for EIT as well as for many other applications, making integrated rubidium or cesium cells highly desirable. Up to now, most work in the area of confined gas spectroscopy has been carried out with cylindrical photonic crystal (HC-PCF) fiber. Confinement and spectroscopy of gases (Benabid et al., Nature 434:488-491, 2005; Ghosh et al., Phys. Rev. Lett. 94:093902, 2005), generation of nonlinear amplification (Benabid, et al., Phys. Rev. Lett. 95, 213903, 2005), EIT and saturated absorption spectroscopy in acetylene (Ghosh et al., Phys. Rev. Lett. 94:093902, 2005; Couny et al., Opt. Comm. 263: 28-31, 2006; Thapa et al. Opt. Lett. 31: 2489-2491, 2006), and signatures of quantum interference in rubidium vapor (Ghosh et al., Phys. Rev. Lett. 97:023603, 2006) have been demonstrated and are indicative of the promise and rapid progress in this field. The HC-PCF-based approach has many advantages, in particular low waveguide loss and resulting long interaction lengths. However, it also has limitations such as the current requirement for attaching rubidium reservoirs connected to a vacuum pump system at the open ends of the HC-PCF (Ghosh et al., Phys. Rev. Lett. 97:023603, 2006) thereby preventing full integration and leaving the complete apparatus large. Another characteristic is the restriction of optical confinement and interaction to one dimension.
Guiding light through hollow optical waveguides has opened photonics to investigating non-solid materials with the convenience of integrated optics. Of particular interest is the confinement of atomic vapors such as alkali vapors, due to the wide range of applications including slow and stopped light (Lukin, M D, Rev Mod Phys 75:457-72, 2003), single-photon nonlinear optics (Schmidt, H. and Imanoglu, A., Opt Lett 21:1936-1938, 1996), quantum information processing (Eisaman M D et al., Nature, 438: 837-841, 2005), precision spectroscopy (Hansch, T. W. et al., Phil. Trans. Royal Soc. London A 363: 2155-2163, 2005), and frequency stabilization (Danielli, et al., Opt. Lett. 25: 905-907, 2000). A need exists in the art for an integrated platform to enable precision atomic or molecular spectroscopy that combines the advantages of photonic crystal-like structures with integrated optics.